mi servirebbe una cosa così

$username = $_SESSION["username"];

$azienda = "'SELECT azienda FROM users WHERE username='$username'";

$sql = "SELECT markers.id, DATE_FORMAT(markers.ts,'%e-%c-%Y %h:%i %p') AS ts, markers.addresstre, markers.addressquattro, markers.addresscinque, markers.addresssei, markers.azienda, comments.idcomment, comments.comment FROM markers LEFT JOIN comments ON markers.id = comments.idcomment WHERE markers.azienda = '$azienda' ORDER BY markers.ts DESC";